1. Field of the Invention
The field of the invention is that of methods of recognizing that a mobile terminal is on a predetermined path to be taken. It relates in particular to methods using transmissions from fixed stations employed in mobile telephone networks.
2. Description of the Prior Art
Calculating the location (the geographical coordinates, for example the latitude and longitude) of a mobile terminal using fixed stations of a radio network and then comparing that location to that of the area in which the mobile terminal is situated is known in the art.
The least sophisticated prior art solutions offer an accuracy of no better than 300 meters on average. These solutions merely recognize a cell of a network within which the mobile terminal is situated (i.e. Cell-ID). The most advanced prior art solutions, such as the solution based on the observed time difference (OTD) technology, offer an accuracy of no better than 50 meters. These technologies take account of a latency time difference between connections to the various radio stations accessible from the current location of the mobile terminal. These solutions have the drawback of necessitating major modification of the network, however.
Also known in the art are solutions, used in particular in air and marine navigation, in which radio transmitters in the form of beacons and dedicated onboard units determine the location of the onboard unit. These solutions include goniometric triangulation, radio compasses or TACANs, and satellite positioning systems such as the Global Positioning System (GPS) and NAVSTAR.
The document DE 101 18 777 discloses a method of determining the position of a mobile radio unit using a mobile radio network comprising a multitude of radio stations that transmit radio signals that the mobile radio unit is able to receive. The method includes a preliminary step of measuring signals from a plurality of radio stations at a multitude of locations whose geographical positions are known accurately. These measurements are typically effected by a service provider and cover the whole of a town systematically. Each measurement yields a receive level/frequency profile which is stored in a profile databank in which position information is associated with each profile. During an operating phase, a mobile radio unit situated at a location whose position is to be determined receives the signals from a plurality of radio stations and measures a station profile depending on its location. The position is then determined by comparing the station profile received with the reference station profiles in the databank.
The above location method can be used to determine if a mobile terminal is on a predetermined path or not: the position of the terminal, as determined by the above location method, is compared electronically with an electronic map of the predetermined path consisting of the stored positions of a series of points constituting the path.
The above prior art method has the drawback of being very costly to implement, since it implies making receive level/frequency measurements systematically at a very large number of points (typically covering the whole of a town) and accurately storing the position of the location used to make each measurement. It also necessitates an accurate knowledge of the positions of a series of points constituting the predetermined path, as obtained, for example, by traveling along the path with a GPS receiver capable of storing a series of positions.